power supply popped

[energised]

I have an HDMI switch which uses a 12V 3A power adapter to feed the power supply board. From the power supply board 2 x 5V feeds go the mainboard. Each one has a 4-pin connector (one red and three black wires on each, looks like an FDD power connector). I measure 12V to the power board but the 2 x 5V show zero volts. I want to feed the two x 5V connectors on the mainboard from another power source. So I would like to run 2 x 5V from an independent power source ...but how do I know the number of A or mA each 5V is supposed to supply the mainboard? It does not say on the board.

 

[chuckey]

It would be easier to mend the first PSU. Failing that, feed the PCBs from a lab PSU that will allow you to measure the current.
Frank